n = input
m = 0
if 999 < int(n) > 10000:
    for left in range(4):
        for right in range(left, 4)
            num = int(n[left:right+1])
            count = 0
            for i in range(2, num//2+1):
                if(num % i == 0):
                    count = +1
            if count == 0:
                if m<num and num != 1:
                    m = num
   if m != 0:
       print(m)
   else:
       print(0)
else:
    print(0)
    
